Classic tale. The dash shell is made of aluminum to save weight. There's a thin foam layer and then vinyl covered. Over time, the vinyl loses elasticity due to solar exposure and then succumbs to the lateral expansion/contraction of the aluminum shell when going through hot and cold exposure.

The reasonable thing to do for those with dashes that haven't cracked yet is to use minimize solar UV exposure via dash covers, windshield screens, UV protectors like Aerospace 303.